GRADUAL EXHAUSTION OF A WHEAT SOIL. 177 



a number of years ; but tlie period wlien this field will 

 cease to give remunerative crops will in that case come 

 all the sooner. 



If we take three wheat fields, and cultivate wheat upon 

 the one, potatoes and clover upon the other two ; and 

 suppose we remove the corn alone from tlie wheat field 

 and heap upon it and plough in all the crop of clover 

 and all the potato tubers, then the wheat field will be 

 more fertile than before, for it has been enriched by all 

 the mineral constituents which the two other fields had fur- 

 nished to the potatoes and the clover. It has received three 

 times as much phosphoric acid and twenty times as much 

 potash as was contained in the corn crop it produced. 



This wheat field will now be able to produce three fuU 

 corn crops in three successive years, because the con- 

 ditions for the formation of straw have remained unaltered, 

 while those for the production of grain have been in- 

 creased three-fold. If the farmer by this method raises 

 as much corn in three years as he could obtain from the 

 same fields in five years without the addition and co- 

 operation of the constituents contained in the clover and 

 the potatoes, it is clear that his profit has been greater, 

 since with three seed-corns he has obtained as good a 

 harvest as in the other case with five. But what the 

 wheat field has gained in fertility, the other two fields 

 have lost ; and the final result is, tliat at less cost of 

 cultivation, and with more profit than before, his three 

 fields are brought to the period of exhaustion, whicli 

 inevitably results from the continued removal of the 

 mineral constituents in the crops of corn. 



The last case which we have to consider is when tlie 

 farmer, instead of growing potatoes and clover, cultivates 

 turnips and lucerne, wliich by their long penetrating 

 roots extract a great quantity of mineral constituents 



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